1. Field of the Invention
The present invention relates to a driving method and a driving apparatus for a semiconductor laser, and an optical communication method and system using the driving method and apparatus. The present invention also relates to a modulation method for a semiconductor laser that is used as a light source for optical communications, in which a modulation signal for driving the laser is caused to be in-phase with an optically modulated signal radiated from the laser over a wide modulation frequency band. The present invention further relates to a driving method for a semiconductor laser for stabilizing and narrowing the spectral line-width of an oscillation wavelength of the laser even if the frequency of fluctuation of the spectrum of oscillation wavelength or optical frequency is low. Within the spectral line-width, the oscillation wavelength of the laser fluctuates with time.
2. Related Background Art
Conventionally, a distributed feedback laser (DFB laser), that radiates in a single longitudinal or axial mode, has been developed as a light source for optical communications, for example.
In a direct optical intensity or amplitude modulation system, which is presently put into practice using the above laser (also referred to as LD hereinafter), the amplitude of a modulation current required for driving LD needs to be set around several tens of mA. In addition, the bias current point of LD needs to be close to its threshold. As a result, the resonance frequency due to relaxation vibration is low, and hence such LD is unsuitable for high-frequency modulation of more than several Gbps. The phenomenon of such resonance is observed in the high-frequency modulation of LD. Furthermore, the fluctuation of the oscillation wavelength during the modulation of LD is large, leading to the problem of wavelength dispersion of signals during long-distance transmissions through an optical fiber and the problem of crosstalk between channels in wavelength division multiplexing (WDM) communications.
On the other hand, in a direct frequency modulation system, which utilizes modulation current injection into LD, the amplitude of modulation current is small, e.g. about several mA, and the bias current point of LD is above its threshold. Thus, LD is constantly modulated while oscillating. Therefore, a direct frequency modulation system is capable of modulation in a wide bandwidth, and its oscillation wavelength fluctuation is small. Thus, the direct frequency modulation system is promising for use in long-distance transmissions and wavelength division multiplexing communications.
When coherent optical communication is employed to apply that direct frequency modulation system to super long-distance transmissions or high-density optical frequency division multiplexing systems, the spectral line-width of a light source should be narrowed and the oscillation wavelength of the light source should be highly stabilized. Narrowing and stabilization can be achieved by an electrical negative feedback control. Also, in this case, the direct frequency modulation characteristic of LD is utilized. Such techniques for stabilization may also be applied to optical measurements.
The direct frequency modulation characteristics of the laser are, however, degraded in a low-frequency range less than several MHz. This is a considerable problem. For example, when a single-electrode DFB laser as shown in FIG. 1 is modulated with a sinusoidal wave current, the relationship between modulation frequency and modulation efficiency (the amount of an optical frequency shift caused by a change in current of 1 mA) varies with modulation efficiency increasing as modulation frequency enters the low frequency region. as shown in FIG. 2. The relationship between modulation frequency and phase difference (a phase difference between modulation current injected into LD and modulation optical signal emitted by LD) also varies, with phase difference increasing to approximately 180 degrees, as modulation frequency decreases to the low frequency region, as illustrated in FIG. 3. It can be seen therefrom that the modulation efficiency fluctuates in a frequency band range less than several MHz and the phase difference also fluctuates and exibits a reverse or anti-phase (a phase shift by about 180 degrees) response as the modulation frequency decreases from approximately 10 MHz and and approaches zero. The reason therefor is that the laser's direct frequency modulation characteristics result from the superposition of competing physical mechanisms; an anti-phase refractive index variation due to heat (a change in the refractive index due to a change in heat in LD shifts in phase relative to the change in heat by 180 degrees), which has a cutoff frequency around several MHz, and the effect of an in-phase refractive index variation due to carrier density (no phase shift of a change in the refractive index due to a change in carrier density in LD occurs relative to the phase of the change in carrier density), which is flat or unchanged up to the resonance frequency. Since the effect of heat is dominant in a low-frequency band range, the flatness of the modulation characteristic is destroyed as discussed above.
Various problems arise when those adverse characteristics appear. First of all, in the case of frequency shift keying (FSK) in which digital signals are transmitted as a frequency modulation, an optically modulated waveform is phase shifted, causing a transmission error, when a modulation frequency falls below several MHz. FIGS. 4A and 4B show such an example. As illustrated in FIG. 4A, the optically modulated pulse width thins when the modulation pulse width is around 1 MHz, and as shown in FIG. 4B the optical waveform is inverted or in an anti-phase with the modulation signal when the modulation frequency is aroud 100 kHz. Therefore, there is a limit to modulation frequency in a low frequency band range, and a freedom in coding is therefore restricted.
Furthermore, when the spectral line-width of LD is narrowed by applying an electric negative feedback thereto, the phase of a change in the total refractive index shifts from the desired if its oscillation wavelength fluctuation has a frequency less than several MHz. Hence the negative feedback control of LD becomes difficult to achieve.
Several methods for improving a semiconductor laser apparatus have been proposed to solve those problems. For example, a three-electrode structure is built as shown in FIG. 5 and a current, injected through end electrodes or central electrode 1045, is modulated to suppress the effect of heat mentioned above (see Yuzo Yoshikuni, et al., IEEE J. Lightwave Technol., vol. LT-5, No. 4, 516, April, 1987). In this case, however, its behaviour varies depending on the amount of bias current injected through each electrode, and variation among devices exists. Thus, such a structure is difficult to put into a practical use. In FIG. 5, reference numeral 1041 designates a substrate, reference numeral 1042 designates an active layer, reference numeral 1043 designates a clad layer, and reference numeral 1044 designates a grating.
Another conventional device has a .lambda./4-shifted diffraction grating 1052, with the depth of the diffraction grating 1052 at a central portion 1056, being larger than that at peripheral portions as shown in FIG. 6. In this device, current injected through a central electrode 1055 is modulated, causing the effects of carrier density mentioned above to exhibit an anti-phase response so as to be in-phase with the effects of heat. As a result, the phase shift of the change in the total refractive index from the desired is eliminated, and further the effect of heat can be reduced. Thus, the characteristic in a low modulation frequency band range is improved (see H. Shoji, et al., Informal Paper of 1992 Vernal Conference of Japan Applied Physics Society, 30a-SF-8). This device, however, has the problem that its manufacturing is complicated, leading to poor yield and a high manufacturing cost. In FIG. 6, reference numeral 1051 designates a substrate, reference numeral 1053 designates a light guide layer, and reference numeral 1054 designates an active layer.